In a sewage pumping station, it is necessary to provide the pump with at least one activation switch for turning power to the pump on and off, depending on the fluid level in the sewage basin. The sewage and effluent which enters the basin contains solids and non-solubles, such as greases. If these materials enter the activation switches, they can render the switch inoperable, resulting in failure of the pump to properly control liquid level in the basin. It is known to provide an isolation tube in the basin. Such an isolation tube is positioned in the basin with its axis essentially parallel to the longitudinal axis of the basin and seated on the bottom of the basin. A small opening provided at the lower end of the isolation tube allows liquid communication between the inside and the outside of the isolation tube, so that the liquid level inside the tube tracks that outside the tube. By maintaining a certain minimum level of liquid in the basin, as through setting the off level of the pump at a minimum level above the basin bottom, the opening in the isolation tube may be kept below the liquid surface at all times. By doing this, the entry of floating solids and non-solubles into the interior of the isolation tube may be generally prevented. However, the isolation tube is not the perfect solution and there are known failure modes.
For example, non-solubles such as grease which enter the basin are not always solid or even semi-solid and may be broken up into small globules by the turbulent flows present in the basin from time to time. The turbulent pull of such globules below the liquid surface may result in entry into the isolation tube interior though the opening. Although an isolated entry of a small amount of the grease into the isolation tube may not be a major problem by itself, the general lack of turbulence within the isolation tube and the relative proximity of the surfaces will often result in the greases being coated out on the walls of the tube and on the switch elements. If this grease builds up through repeated occurrences, the switch elements may become inoperable. Also, small solids are easily entrained in the same turbulences and drawn into the isolation tube, where they may become trapped, if the aperture connecting the interior and exterior of the isolation tube is large. Repairs of the level control switch elements due to contamination are costly and inconvenient.